The present invention is directed to photoelectric smoke detectors and in particular to photoelectric smoke detectors having a smoke detection chamber with generally uniform smoke penetration properties around its entire periphery.
Smoke detectors based on light scattering by smoke particles have been known for a number of years. Such detectors at present utilize solid state components including photoemitting diodes and photodiode detectors, the two devices being incorporated in a fixed mounted arrangement within a smoke detection chamber. The smoke detection chamber is generally designed to exclude most ambient light influences while providing openings to permit entry of the smoke particles into the detection chamber.
There have been many designs of light scattering smoke detectors developed and patented which rely upon the walls of the smoke chamber having labyrinth designs to allow the passage of the smoke particles while excluding ambient light from the interior of the chamber. Examples of such designs are shown in U.S. Pat. Nos. 3,914,616, 4,168,438, 4,216,377, 4,672,217, 4,758,733, 5,138,302, 5,400,014, 5,430,307, 5,543,777, 5,546,074, 5,552,765 and 5,642,099.
In many of the chambers of the above noted patents, the photoemitting diodes and photodiode detectors are mounted in either the sides of the chamber or on the bottom of the chamber, most typically with a 60xc2x0 scattering angle along a horizontal plane between the photoemitting diodes and photodiode detectors. The design of many of the prior art smoke detection chambers results in a generally horizontal flow of the smoke particles through the chamber. Ideally, the photoemitting diode and photodiode detector should be mounted in such a way that the intersection of the transmitted light from the photoemitting diode and the view of the photodiode detector falls within the horizontal path of the smoke particles. However, depending upon the direction of the smoke particles and their laminar flow rate through the chamber, the horizontal flow may be shifted from the intersection thereby affecting the sensitivity of the smoke detector. Also, the use of the 60xc2x0 scattering angle increases the distance between the photodiode detector and photoemitting diode thereby affecting the sensitivity and increasing the potential for dust particle interference.
Smoke detectors utilizing a generally perpendicular scattering angle have greater sensitivity as it is possible to mount the photoemitting diode and photodiode detector closer to each other to increase the amount of light in the detection zone. Such designs generally require the photoemitting diode to be mounted in the side wall of the smoke detection chamber. Examples of such smoke detectors are shown in U.S. Pat. No. 3,914,616, and the applicants previous U.S. Pat. No. 5,719,557.
In both the 90xc2x0 scattering angle designs as well as a number of the 60xc2x0 scattering angle designs, one or both of the photoemitting diode and photodiode detector are mounted in the side wall. The mounting of one or both of the elements in the side wall of the smoke detection chamber results in a large solid area in the side wall which acts as a block and does not permit easy passage of smoke particles into the chamber. In order for the smoke particles to enter the chamber, they have to go around the solid area block. Depending upon the size of such solid area blocks, there can be a 20% or more difference in sensitivity of the smoke detector when the direction of the smoke flow is at the region of the solid area block compared to when the smoke direction is from the side which does not have such a solid area block.
In order to achieve optimum detection of smoke particles in a fire situation, the smoke detector should be responsive to the presence of smoke from any direction. Variations in responsiveness to smoke from different directions can cause a delay in the annunciation of an alarm condition by the smoke detector. This could result in the fire condition being more advanced when the alarm is given and could result in the occupants of the space in which the smoke detector is located having less time in which to vacate the space.
One way some of the prior art detectors have tried to alleviate the problem of variations in responsiveness is by making the side wall of the chamber less xe2x80x9copenxe2x80x9d throughout its periphery. For example, U.S. Pat. Nos. 4,216,377 and 4,672,217 both illustrate smoke detection chambers having xe2x80x9cscoop finsxe2x80x9d with very large legs and small spaces between the legs. Similarly, U.S. Pat. Nos. 4,758,733, 5,138,302, and 5,546,074 all describe smoke detection chambers in which a significant portion of the area of the side wall is filled by labyrinth wall elements resulting in reduced open areas through which the smoke particles can pass.
Another problem faced by photoelectric smoke detectors relates to the nature of the smoke particles to be detected. Smoke is generally classified as black or gray. Gray smoke particles are generally much easier to detect as they tend to scatter the light from the photoemitting diode very well. Hence, most designs of photoelectric smoke detectors are reasonably effective at detecting gray smoke. Black smoke particles, on the other hand, do not generally scatter the light as well and many designs of photoelectric smoke detectors have difficulty properly detecting the presence and level of black smoke. This is particularly the case with those detectors utilizing a 60xc2x0 scattering angle as, at this angle, the gray smoke to black smoke sensitivity is only 4:1. In these detectors which are usually set to detect gray smoke at about 3% per foot obscuration, the level of black smoke required to indicate an alarm state would be 12% per foot obscuration or higher. Thus, there still exists a need to provide very sensitive smoke detection of both black and gray smoke particles with generally uniform responsiveness to smoke from any direction.
The present invention in one aspect provides for a photoelectric smoke detector comprising a case having mounted therein a circuit board and a smoke detection chamber. The smoke detection chamber has a side wall, a top and a bottom. The side wall is provided with a double row of generally rectangular vanes arranged to provide a labyrinth extending generally around the entire periphery of the smoke detection chamber for ingress and egress of smoke particles. A photoemitting diode is mounted in a mounting arrangement in the side wall of the smoke detection chamber so that a light beam from the photoemitting diode is transmitted across the smoke detection chamber. The mounting arrangement extends from and is spaced from the bottom on a narrow strut to provide minimal interference for entry of smoke particles. The bottom of the smoke detection chamber has an opening therein with a shielding arrangement thereabout open to the chamber. The circuit board is mounted to overlie the bottom surface and includes a photodiode detector mounted directly thereon and positioned generally in the opening so that it views into the smoke detection chamber through the opening and the shielding arrangement, the shielding arrangement shielding the photodiode detector from incident light which may be present in the smoke detector chamber while providing for a diverging field of view of the photodiode detector which intersects the light beam of the photoemitting diode to define a detection volume contained within the smoke detection chamber. The double vane side wall and spacing of the mounting arrangement from the bottom provides the smoke detection chamber with generally uniform smoke penetration properties around its entire periphery.
In another aspect of the invention there is provided a smoke detection chamber for use in a photoelectric smoke detector. The smoke detection chamber comprises a side wall, a top and a bottom. The side wall is provided with a double row of generally rectangular vanes arranged to provide a labyrinth extending generally around the entire periphery of the smoke detection chamber for ingress and egress of smoke particles. A mounting arrangement for a photoemitting diode is provided in the side wall of the smoke detection chamber so that a light beam from a photoemitting diode is transmitted across the smoke detection chamber. The mounting arrangement extends from and is spaced from the bottom on a narrow strut to provide minimal interference for entry of smoke particles. The bottom of the smoke detection chamber has an opening therein with a shielding arrangement thereabout open to the chamber for a photodiode detector. The double vane side wall and spacing of the mounting arrangement from the bottom provides the smoke detection chamber with generally uniform smoke penetration properties around its entire periphery.
In yet another aspect of the invention, the smoke detection chamber is provided as a bottom having the opening and shielding arrangement and a vane extending upwardly to which is attached the mounting arrangement for a photoemitting diode and a top having the double row of rectangular vanes extending downwardly therefrom, the top with the downwardly extending vanes being releasably attachable to the bottom to form the smoke detection chamber.